Vacuum furnaces for the plasma carburisation of metallic workpieces by means of a carbon-containing gas, for example methane or propane, are known. During the plasma carburisation, the workpieces are heated in the vacuum furnace to a temperature of between about 800° C. and 1050° C. Subsequently, the carbon-containing process gas is led into the furnace chamber and an electric field is applied to the workpiece batch. Thereafter, for the purpose of hardening, the batch is cooled by blowing it with cooling gas emerging from nozzles onto the batch, helium, in particular, having proved successful as the cooling gas.
By way of example, a vacuum shaft furnace is known (DE 32 08 574 A1) which has a device for cooling the heat-treated batch by means of a gas flow guided through the interior of the preferably cylindrical heating chamber via openings and circulated outside the heating chamber in the closed furnace housing by a gas blower via a gas cooler, closable openings being arranged in the floor and the roof for a vertical flow in the heating chamber, and closable openings lying one above the other being arranged at opposite locations in the side wall of the heating chamber over its entire height for a horizontal flow. To close the openings in the wall of the heating chamber there are provided cover plates which cover all the openings, each contain openings congruent with the openings in the side walls and are displaceable by half an opening spacing.
Furthermore, a vacuum furnace for the plasma carburisation of metallic workpieces is known (EP 0 535 319 B1) which has an electrical heater, a vacuum pump for generating a vacuum in the heating chamber, and gas inlet openings, by means of which cooling gas conveyed by a blower and guided via a heat exchanger is fed to the batch, the gas inlet openings which guide the cooling gas being arranged in the heating chamber and aligned with the batch. The nozzles designed as gas inlet openings are arranged all around the heating chamber and at the ends, the end nozzles serving to introduce the cooling gas axially into the heating chamber. A vacuum furnace for plasma carburisation designed in such a manner makes it possible to harden the carburised batch to complete the heat-treatment process, without having to remove the batch from the heating chamber to do so. Since all the heat-treatment steps can be performed exclusively within a heating chamber, the space requirement of a single furnace is also relatively small. Since the gas guidance and the gas flow are crucial factors for the quenching process, but a reversal of the flow direction of the cooling gas cannot be accomplished with the aforementioned vacuum furnaces, it has also been proposed to equip the furnace housing with two chambers separated from each other by a closing slide and to arrange the heating elements and a hot-gas fan in one chamber and the cooling fan and the heat exchanger with suitable flow plates in the other chamber. With this type of furnace, the batch is firstly heated up and carburised in one chamber and then, with the closing slide open, moved into the other chamber for the purpose of quenching.
The most fundamental disadvantage of all known vacuum furnaces, however, is that renewed charging of the furnaces is only ever possible after total completion of the previous treatment process in each case, and this means, where large-scale manufacture is required, setting up a large number of complete vacuum furnaces. Since, however, the first phase of the heat-treatment process, namely the heating-up and carburising of the batch, takes a relatively long time compared with the second phase, namely the hardening process, the object on which the present invention is based is to provide an apparatus for the treatment of metallic workpieces with cooling gas which avoids the disadvantages of known furnaces and with a very compact construction—with a low ratio of chamber volume to batch volume—enables a rapid flow reversal, and in which mirror-symmetrical flow conditions exist after the flow reversal. Furthermore, the apparatus is to be of single-walled design and, from the very beginning of the quenching phase, enable a high heat transfer coefficient at all the workpieces within the batch, is to require a small amount of quenching gas per quenching operation and permit operation with particularly short cycle times. Finally, the apparatus is to be designed so as to enable controlled quenching—i.e. with variable intensity—and to be capable of being coupled to existing carburising furnaces, so that a plurality of simple furnaces—without heat exchanger and cooling-gas blower—can be operated with a single apparatus, which reduces costs and saves space.
This object is achieved according to the invention by an apparatus having a housing for introducing and removing the workpieces, having a cooling-gas source, by means of which blower-conveyed cooling gas guided via heat exchangers is fed to the workpieces, having a workpiece support with support plates arranged on both sides of the workpiece support, extending vertically and parallel to one another, separating the workpieces from lateral spaces and provided with openings, and having heat exchangers held, above and/or below the workpieces, between the support plates, and having blower motors, provided on both sides of the housing, with shafts extending into the lateral spaces horizontally and at right angles to the axis of the housing, the blower wheels, which rotate with the shafts in blower housings, each being provided close to the inner wall of the housing and being separated from the support plates by air-guiding plates which, each held by the blower housings, extend parallel to and at a distance from the support plates and with the support plates each form, on both sides of the workpiece batch, two vertically extending shafts for guiding the cooling-gas stream, reversing flaps being mounted at each of the upper and lower ends of the two air-guiding plates and, depending on the position, sealingly butting against the support plates or against the inner wall of the housing.
Further details and features are described in more detail below.
The invention permits a wide variety of possible embodiments; one of these is illustrated in the appended drawing, which shows an apparatus purely schematically in cross-section.